Gas liquefaction is achieved by cooling and condensing a feed gas stream against multiple refrigerant streams provided by one or more recirculating refrigeration systems. Cooling of the feed gas is accomplished by various cooling process cycles such as the well-known cascade cycle in which refrigeration is provided by three different refrigerant loops. In the liquefaction of natural gas, for example, a cascade refrigeration system may be utilized with methane, ethylene and propane cycles in sequence to produce refrigeration at three different temperature levels. Another well-known refrigeration cycle uses a propane pre-cooled, mixed refrigerant cycle in which a multicomponent refrigerant mixture generates refrigeration over a selected temperature range. The mixed refrigerant can contain hydrocarbons such as methane, ethane, propane, and other light hydrocarbons, and also may contain nitrogen. Versions of this efficient refrigeration system are used in many operating liquefied natural gas (LNG) plants around the world.
Another type of refrigeration process for natural gas liquefaction utilizes a gas expansion cycle in which a refrigerant gas such as nitrogen is compressed and cooled to ambient conditions with air or water cooling and is further cooled by countercurrent heat exchange with cold low-pressure nitrogen gas. The cooled nitrogen stream then is work expanded through a turbo-expander to produce the cold low-pressure nitrogen gas, and this gas is used to cool the natural gas feed and the compressed nitrogen stream. The work produced by nitrogen expansion can be used to drive a nitrogen booster compressor connected to the shaft of the expander. In this process, the cold expanded nitrogen is used to liquefy the natural gas and also to cool the compressed nitrogen gas in the same heat exchanger. The cooled pressurized nitrogen is further cooled in the work expansion step to provide the cold nitrogen refrigerant.
Integrated refrigeration systems can be used for gas liquefaction wherein cooling of the gas from ambient to an intermediate temperature is provided by one or more vapor recompression cycles and cooling from the intermediate temperature to the final liquefaction temperature is provided by a gas expansion cycle. Examples of these combined liquefaction cycles are disclosed in German Patent DE 2440215 and in U.S. Pat. Nos. 5,768,912, 6,062,041, 6,308,531 B1, and 6,446,465 B1.
In the processes described in DE 2440215 and in U.S. Pat. Nos. 5,768,912 and 6,446,465 B1, feed gas and compressed refrigerant gas from the gas expansion cycle are cooled together in common heat exchangers using refrigeration provided by the cold work-expanded refrigerant. In an alternative method disclosed in U.S. Pat. No. 6,308,531 B1, feed gas and compressed refrigerant gas from the gas expansion cycle are cooled in separate heat exchangers using refrigeration provided by the cold work-expanded refrigerant. In this method, additional refrigeration from the vapor recompression cycle is used to provide additional cooling of the compressed refrigerant gas in the gas expansion cycle. This may be accomplished by passing a stream of refrigerant from the vapor recompression cycle through the heat exchanger cooling the compressed refrigerant gas. Alternatively, a portion of the gas expansion cycle compressed refrigerant gas may be cooled against vaporizing refrigerant in the vapor recompression cycle heat exchangers to provide additional refrigeration.
The liquefaction of natural gas is a very energy intensive process. Improved efficiency and operating flexibility of gas liquefaction processes using combined vapor recompression and gas expansion refrigeration cycles are highly desirable and are among the objectives for new cycles being developed in the gas liquefaction art. Embodiments of the present invention address this need by providing multiple expanders in the gas expansion cycle to reduce or eliminate the need for balance refrigeration between the vapor recompression and gas expansion cycles while allowing cooling of the feed gas and the compressed gas expansion refrigerant in separate heat exchangers and also allowing independent operation of the vapor recompression and gas expansion cycles.